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Labelling-Determination of the energy content of food

Details

The purpose of this section is to provide some advice on how the nutrient density or energy content of foods is calculated and displayed on food labels. A calculator is also included to enable students producing new products to calculate the energy density using the chemical constituents of the food. The calculator can also be used as a food calorie calculator.

The salt content of foods is receiving increasing attention by government and consumer groups and information on salt is now available on labels. I will try to include some information on salt from a labelling perspective in the future. However, the Dairy Science and Food Technology site does contain some general information on salt in the FAQ's section. There is also a calculator for converting sodium to salt and determining the salt contribution from an individual meal to the recommended maximum daily intake for infants, children and adults.

How is energy content recorded on labels?

In the UK and other member states of the European Union manufacturers of pre-packaged food must label the nutritional energy of their products in both kilocalories ("kcal") and kilojoules ("kJ"). The energy content of food is generally given for 100 g and for a typical serving size. In countries and in situations where calories are used, kilocalories rather than calories are quoted on labels.

How is energy content of a food calculated?

Classically the total energy content of a food was determined by burning the food in a bomb calorimeter and measuring the amount of energy released. Carbohydrates, fats and protein are the major energy sources in foods although alcohol, organic acids and polyols may be important in particular foods.

Polyols have been called sugar alcohols but are neither sugars nor alcohols. They should be considered as reduced energy sugar-replacers, sweeteners and are included in total carbohydrates. Currently used polyols include sorbitol and erythritol.

Note that the energy values of different food fibres and polyols show significant variation. Consequently if a food contains high concentrations of a specific fibre or polyol then generic energy conversion values should be replaced with the specific conversion factors.

The energy information on labels is determined by analysing the chemical composition of a food, the concentrations of protein, fat, carbohydrate for example and adding the energy value of the components to give a total value. Note that methods for determining carbohydrates, including fibre, in food are still the subject of discussion and methods are subject to on-going modification. For more information please see the IFST and Food Standards websites and the report Food energy - methods of analysis and conversion factors that can be downloaded from the FAO website

Where can food manufacturers and others obtain the nutrient composition of foods? Extensive nutritional data for most foods can be obtained from several well characterised and authenticated sources. One that is particularly easy to access is the United States Department of Agriculture's 'National Nutrient Database for Standard Reference . The standard UK reference work is McCance and Widdowson's 'The Composition of Foods'. This reference was updated in 2002 and is now in its sixth edition.

The human body is not as efficient as a bomb calorimeter in converting food nutrients to energy and this is allowed for in calculating nutrient density. So-called coefficients of digestibility have been determined by several groups and there is general acceptance that humans can utilise the energy in some 97% of carbohydrates, 95% of fats, and 92% of proteins in foods. In general these coefficients are used to adjust the data from bomb calorimetery to obtain more meaningful values for use with people. These coefficients are referred to as Atwater general factors and while they still have utility consideration is being given to their replacement see for example Food energy - methods of analysis and conversion factors. Useful data for determining the energy density of foods are shown in table 1; note that these are based on the Atwater factors.

Note that total carbohydrate and available carbohydrate are different. Total carbohydrate represents the sum of all the 'carbohydrate-like components in the food. It is frequently obtained by subtracting protein, fat, ash, alcohol and water from 100. Dietary fibre, polyols and organic acids will also be included in this value.

Available carbohydrate is carbohydrate that is readily available (without fermentation) for metabolism and does not include dietary fibre.

Calculate the energy value of a food

The energy calculator developed by the author can be used to give a good approximation of the energy content of most foods. The calculator will overestimate energy in foods containing high concentrations of dietary fibre. The nutrient composition of foods as discussed previously can be obtained from either McCance and Widdowson's book or the USDA's 'National Nutrient Database for Standard Reference . Note there are climate, varietal and other factors that contribute towards differences in the nutrient density of fruits and vegetables. Whether these are sufficiently different to require the use of local databases for labelling purposes, given the natural variation in nutrient density within fruits and vegetables, requires informed judgment. Feeding regime, animal genetics, climate and systems of animal husbandry also affect the nutrient density of meat and milk and consequently products derived from them. Hence the need for caution in the use of nutritional data and why there is a general preference to use local databases.